White light emitting LEDs (“white LEDs”) are known and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example, in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photo-luminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit light of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light, green and orange or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor material combined with the light emitted by the phosphor provides light which appears to the eye as being nearly white in color.
The phosphor typically resides within a wavelength conversion layer, in which phosphor material formed is mixed with a light transmissive carrier material, typically a plastics material. The wavelength conversion layer is operable to absorb a proportion of the blue light generated by the LEDs and convert it to light of a different wavelength by a process of photoluminescence. The combination of the different wavelength light produced by the phosphor-based wavelength conversion layer (e.g., yellow light) combined with the residue blue light that passes through the wavelength conversion layer forms light that appears white to the human eye.
A problem with existing wavelength conversion components is the degradation of the wavelength conversion layer when exposed to external environmental conditions. As noted above, the wavelength conversion layer is typically composed of a mixture of a phosphor material and a plastics carrier material. When the plastic-based wavelength conversion layer is exposed to moisture (e.g., mixture of air and water), oxygen, or other environmental contaminants, light energy being absorbed by the wavelength conversion layer may cause the contaminants to initiate chemical reactions with the phosphor material leading to accelerated degradation of the wavelength conversion layer.
The effect of water absorption on photoluminescence varies between phosphor compositions and can be more pronounced for silicate-based phosphor materials which are able to more readily form water soluble compounds. The absorption of water can occur even when the phosphor material is encapsulated in a polymer binder (e.g., carrier material/binding material), such as silicone, and a reduction in light emission of ˜10% may occur for a device with an ortho-silicate phosphor that is operated in a humid environment (i.e. ≧80% relative humidity) at a temperature of 25° C. for more than 200 hours.
Moreover, an exposed wavelength conversion layer may be prone to handling damage, such as surface scratches, which also degrade the performance and lifetime of the wavelength conversion layer over time.
Therefore, there is a need for an improved approach to implement photo-luminescent materials for a lighting apparatus which addresses these and other problems with the prior implementations.